1. Field of the Invention
The present invention relates to a switching power supply apparatus that includes a transformer and two switching elements.
2. Description of the Related Art
A switching power supply apparatus of the related art that is configured such that two switching elements are alternately switched on and off in a complementary manner is described in Japanese Unexamined Patent Application Publication No. 2001-37220 and Japanese Unexamined Patent Application Publication No. 7-274498.
FIG. 1 is a circuit diagram of the switching power supply apparatus described in Japanese Unexamined Patent Application Publication No. 2001-37220. In FIG. 1, a switching power supply apparatus 1 employs a circuit that is generally called a flyback converter. A main switching element Q1 is repeatedly alternately switched on and off, and when the main switching element Q1 is switched on, the transformer 1 accumulates energy, and when the main switching element Q1 is switched off, electricity is supplied to a load. In addition, in the switching power supply apparatus 1, a so-called active clamping method is employed in which a surge voltage applied to the main switching element Q1 is clamped and a zero-voltage switching operation of the main switching element Q1 and a sub switching element Q2 is realized.
In the switching power supply apparatus 1, a FET Q1, which serves as the main switching element, a primary winding N1 of a transformer T, and a direct current power supply E are connected in series with one another, and a series circuit formed of a FET Q2, which serves as the sub switching element, and capacitor C1 is connected between the two ends of the primary winding N1 of the transformer T.
Here, the gate of the FET Q1 is connected to an end of a first driving winding N3 via a main switching element control circuit (main control circuit) 2. In addition, the source of the FET Q2 is connected to the drain of the FET Q1 and the gate of the FET Q2 is connected to an end of a second driving winding N4 of the transformer T via a sub switching control circuit (sub control circuit) 3.
In addition, the gate and the source of the FET Q2 are connected between the two ends of the second driving winding N4 via the sub control circuit 3. The sub control circuit 3 includes a transistor Q3, a capacitor C2, a resistor R1, a capacitor C3, a resistor R2 and an inductor 4. Within this circuit, the capacitor C2 and a resistor R1 form a time constant circuit.
Furthermore, the switching power supply apparatus 1 includes a rectifying diode Do and a smoothing capacitor C4 on the secondary side of the transformer T.
On the other hand, a switching power supply apparatus in which both low-side and high-side switching elements are driven using an IC is disclosed in Japanese Unexamined Patent Application Publication No. 7-274498.
The switching power supply apparatus of Japanese Unexamined Patent Application Publication No. 2001-37220 is configured such that a high-side switching element Q2 is driven using the driving winding N4 of the transformer T and the time constant circuit, and is realized using a simple circuit. However, in control in which the frequency is changed, it is preferable that the on period of the switching element Q2 also be changed, but since the on period of the switching element Q2 is substantially fixed by the time constant circuit, making such a corresponding change is difficult.
On the other hand, as in the switching power supply apparatus of Japanese Unexamined Patent Application Publication No. 7-274498, in a configuration in which both the low-side and high-side switching elements are driven by an IC, setting of a time ratio and setting of the frequency can be performed comparatively easily. However, when there is a period in which the ground potential of the high-side switching element is the same potential as the power supply voltage, an IC designed to withstand high voltages is required since direct current driving will be performed by the IC. The design of an IC designed to withstand high voltages is complex and a very expensive IC has to be used. Furthermore, in a configuration in which two switching elements are directly driven by a single IC, wiring of peripheral circuits around the IC need to be connected to both of the two switching elements and therefore the peripheral circuits also become more complex. Accordingly, it is inevitable that the switching power supply apparatus will be increased in cost.